First, the US should back the partition of Syria into a Sunni majority state and an Alawite rump state in the northwestern quadrant of the country, where the Russian navy station happens to be located. The Kurds should get autonomy, just like their Iraqi compatriots.
Turkey will object vociferously because it would advance Kurdish independence, which Prime Minister Recep Tayyip Erdogan views the way Captain Hook viewed the crocodile. Too bad for the Turks: someone has to lose here, and it might as well be they. Partition is the only way to stop the civil war and avoid mass murder in its wake. Total victory by either side would be followed by massacres. The most humane solution is a breakup on the precedent of the former Yugoslavia. Assad can remain in power in a rump state where the Alawites will be safe from Sunni reprisals, and the Russians can keep their fueling station. One wonders why the “responsibility to protect” crowd in Washington hasn’t considered that.
Second, the US should use its influence with Turkey, Saudi Arabia and Qatar to clean out the nastier jihadist elements among Syria’s Sunni rebels. It should also make clear to the Russians that it will not interfere with their counter-terrorist operations in the Caucasus, grisly as these might be.
Third, the US should attack Iran and destroy its nuclear weapons capability and key Revolutionary Guard bases (and perhaps a few other things; various American flag officers have they own list of druthers).
Neutralizing Iran is the key: it eliminates the pipeline of support from Iran to Assad and various terrorist organizations, and reduces them to obnoxious but strategically unemployed local players.
Russia evidently has fewer objections to an American air strike on Iran than on Damascus. It has signaled this as clearly as it can by refusing to deliver the S300 system to the Iranian regime while promising to deliver it to the Syrian regime. The bad news is that we cannot extract Russia from the region; America has made too many blunders in the region to turn the clock back.